Since a high-temperature solid-electrolyte fuel cell (SOFC) can execute a power generation process in a high temperature region, it is said that there can be realized not only a distributed power source of several hundred kilowatts but also a highly effective composite-type base load power generation plant combined with a gas turbine and a steam turbine, and having a large capacity of several hundred megawatts, by the fuel cell. A power generation system employing the SOFC is effective in energy conversion into electric energy; it can minimize exhaustion of atmospheric pollutants, and can utilize various fuels, such as a coal gas, a commercial city gas, and the like, by a hydrogen purifying function achieved by internal reforming. For the reasons described above, research and development of the solid electrolyte fuel cell are proceeding in Japan as well as overseas, in great expectation that it will be developed into a fuel cell operating at high temperature, following the phosphoric acid fuel cell (PAFC) and the molten carbonate fuel cell (MCFC).
The solid electrolyte fuel cell is broadly classified into a flat type and a tubular type. The tubular type is said to be advantageous in that it is excellent in strength and can separate gas relatively easily. Further, the tubular type requires solving fewer technical problems, because a gas is sealed only at the ends, and thus the number of elements to be developed is small. The tubular type is further classified into a longitudinal stripe type (Westinghouse type: a high current type having a structure in which unit cells are connected externally) and a lateral stripe type (a series connection type: a high voltage type having a structure in which unit cells are connected internally) (for example, refer to “Power Generation with a Fuel Cell”, edited by Specialized Committee for Researching Fuel Cell Operability of The Institute of Electrical Engineers of Japan, issued by Corona (1994), pages 76-80). Of these two types, the present invention relates to the lateral stripe type. Since the lateral stripe type employs internal connection, once unit cells are produced, they are advantageous in operation stability as stacked cells.
FIG. 24 is a front elevation view showing a conventional solid electrolyte fuel cell of the lateral stripe tubular type with its upper half portion in cross section. As shown in FIG. 24, a gas-tight portion (non-power-generating portion) and a permeability portion (power-generating portion) are formed by forming a dense ceramic membrane 51 around the circumference of a porous ceramic base tube 50. A fuel cell stack is completed thereafter by sequentially forming a fuel electrode 52, a solid electrolyte membrane 53, an inter-connector 54, a current taking-out terminal lead 55, a dense ceramic membrane 56, and an air electrode 57. The process for producing a lateral stripe tubular SOFC 60 is completed by attaching current taking-out lead wires 58 and gas supply/exhaust ceramic end caps 59 to both sides of the fuel cell stack. As described above, a lateral stripe tubular SOFC has such an arrangement that unit cells, each of which has a three-layer-laminated structure comprised of a fuel electrode, a solid electrolyte, and an air electrode formed on a base tube, are connected in series to each other through an inter-connector, although the materials thereof are somewhat different.
Operation of the solid electrolyte fuel cell will be briefly explained here. Basically, the solid electrolyte fuel cell has a structure in which a solid electrolyte is sandwiched between electrode plates having good gas permeability. Yttria-stabilized zirconia (YSZ), which has a composition of, for example, (Y2O3)0.08(ZrO2)0.92, as a composite oxide that maintains a fluorite-type cubic crystalline structure from room temperature to a high temperature, and which is chemically stable, is used as the solid electrolyte. Since the yttria-stabilized zirconia is composed of quadrivalent zirconium oxide, in which tervalent yttrium oxide is solid-solubilized, an oxide ion vacancy is formed in a crystal, and the vacancy moves freely in the crystal at a high temperature. When gas-permeable electrodes are attached to both sides of the YSZ, and an oxygen density difference is applied to both sides thereof, oxygen moves into the YSZ, as O2− ions from the high density side (the cathode, which is generally referred to as an air electrode), and it is moved to the low-oxygen side (the anode, which is generally referred to as a fuel electrode) by the density difference, to thereby carry electrons. The O2− ions, having reached the anode, react with the fuel and discharge electrons, and the discharged electrons flow in an external circuit, and work on a load.
The advantage of the solid electrolyte fuel cell (SOFC) can be utilized at the maximum by operating it at a high temperature (for example, 800 to 1000° C.). Accordingly, how thermal stress is suppressed to a low level is a significant problem, from which it is said to be preferable for all of the components of the SOFC to be comprised of a combination of materials whose thermal expansion coefficients are in conformity with each other. Thus, an oxide electrically conductive material, such as a lanthanum-chromite oxide (LaCrO3, which contains Mg, Ca, and Ti to adjust a thermal expansion coefficient, is often used), is used as a material of the inter-connector of the SOFC (refer to, for example, JP-A-52-21743 and JP-A-2002-145658). In the longitudinal stripe type (Westinghouse type: high current type), the LaCrO3 material has no particular problem, because an electrically conductive passage is short in an inter-connector portion (the structure of this type is essentially devised to shorten the electrically conductive passage). However, since the lateral stripe type (series connection type: high voltage type) has a long electrically conductive passage in its structure, a problem arises in that the internal resistance of a cell is greatly increased when an oxide material having poor electric conductivity is used. To cope with this problem, it is proposed to compose the inter-connector of cermet; that is, an alloy material having higher electric conductivity, such as NiAl, NiCr or the like (refer to, for example, JP-A-8-162139 and JP-A-8-222246).
The inter-connector is required to secure gas tightness and to have high electric conductivity. Although an electrically conductive material is also used in a fuel electrode and in an air electrode, they are relatively less affected by thermal stress, because they are composed of porous membranes. However, since the inter-connector must be formed of a dense membrane to secure gas tightness, a large amount of internal stress occurs in the inter-connector. In particular, in the lateral stripe tubular type, since the inter-connector has large thickness, when the alloy material is used, there is a large difference between the thermal expansion coefficient of the inter-connector and that of a solid electrolyte, even if the alloy material is made to cermet, and thereby an inter-connector portion is liable to be deteriorated by repeating operation, and stop. That is, exfoliation and cracks are liable to occur in the inter-connector portion. In contrast, even if the inter-connector employs an alloy material as its material, since the alloy material is used as cermet, it is difficult to secure a high conversion efficiency, because the resistance value increases in the cell connecting portion.
Further, another problem of the conventional SOFC resides in that the gas permeability of electrodes is deteriorated while they are operated for a long period of time. Conventionally, when an electrode for an SOFC is formed, it is ordinarily composed of a porous thin membrane having a uniform thickness and formed on an entire electrolyte membrane (on both the front and back surfaces thereof). The membrane has a contradictory condition in that it must be provided with electric conductivity while also be formed as a porous membrane having good gas permeability. Accordingly, when the thickness of the membrane is excessively reduced or is excessively made porous to improve the gas permeability, the problem arises that the electrically conductive performance of the membrane is deteriorated, and the internal resistance of a cell is increased. In contrast, when the thickness of the membrane is increased to secure electric conductivity, gas passage is increased, and thus gas-permeable performance is deteriorated. Therefore, a compromise between both conditions must be made at some intermediate point.
Since the SOFC has a high operating temperature of 800 to 1000° C., and an electrode membrane is exposed to a high temperature for a long time, gas cavities become clogged, due to sintering and the like of the electrode membrane, as time passes, so that a phenomenon arises that the gas-permeable performance is deteriorated. Accordingly, power generation performance is deteriorated, because fuel gas is not supplied well on the fuel electrode side, and oxidizing agent gas is not supplied well on the air electrode side. As a result of measurement, deteriorating characteristics, in which the output of generated power is gradually reduced, often arise. It cannot be said that all causes of this phenomenon are attributable to the above problems, but it is a fact that the problems considerably relate to the causes of the phenomenon. This phenomenon sometimes arises in both the air electrode and the fuel electrode; it sometimes arises in only one of them, and it also arises in association with an electrode material. Accordingly, it is difficult to specify the cause of the phenomenon. Trouble arises in the supply of gas, because the gas-permeable performance is deteriorated due to sintering of the electrodes (air electrode and fuel electrode) for the SOFC.
Another problem of the conventional SOFC resides in that exfoliation and cracks occur in the air electrode and the fuel electrode when they are kept at a high temperature for a long time, because they are composed of a material different from that of the base tube and the solid electrolyte, and thus the thermal expansion coefficient of the electrodes is different from that of the base tube and the solid electrolyte. In addition to the above problem, the possibility that the air and fuel electrodes may be exfoliated is increased, because they have an inferior intimate contact property with the base tube and the solid electrolyte.